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In order to force bacteria… (Class 12 Biology 2026, Q20b)

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BiologyCBSE Class 12 · Board exam 2026, set 57/5/1 · 2 marks · Very short answer

CBSE Class 12 Biology 2026: question 20(b)

Set in the CBSE Class 12 board exam of 2026, paper 57/5/1, section B, question 20(b), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20(b) · 2 marks · Very short answerBiotechnology: Principles and Processes

In order to force bacteria to take up the recombinant DNA, they must be made competent. Explain how it can be achieved.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

DNA is negatively charged. A chemical treatment with a cation helps it enter the bacterial cell.

Hint 2

The chemical method is followed by a cold–hot–cold temperature sequence.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. Treat the bacterial cells with a specific concentration of a **divalent cation such as calcium ()**. This increases the efficiency with which DNA enters through pores in the cell wall.

    ½ mark

  2. Incubate the treated cells with the recombinant DNA on ice.

    ½ mark

  3. Give a brief heat shock at 42 °C.

    ½ mark

  4. Put them back on ice. This enables the bacteria to take up the recombinant DNA.

    ½ mark

Answer: Ca²⁺ treatment, incubation with rDNA on ice, heat shock at 42 °C, then back on ice.

Where the marks are

  • Treatment with divalent cation (½ mark)
  • Incubation with recombinant DNA on ice (½ mark)
  • Heat shock at 42 °C (½ mark)
  • Placing back on ice (½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Biotechnology: Principles and Processes, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 57/5/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.